Although fragile X syndrome (FXS) is caused by a hypermethylated full mutation (FM) expansion with ≥200 cytosine-guanine-guanine (CGG) repeats, and a decrease in <i>FMR1</i> mRNA and its protein (FMRP), incomplete silencing has been associated with more severe autism features in FXS males. The gene discussed is FMR1; the disease is fragile X syndrome.